Single-cell DNA-methylation analysis reveals epigenetic chimerism in preimplantation embryos

TRIM28-dependent developmental heterogeneity determines cancer susceptibility through distinct epigenetic states

Mutations in cancer risk genes increase susceptibility, but not all carriers develop cancer. Indeed, while DNA mutations are necessary drivers of cancer, only a small subset of mutated cells go on to cause the disease. To date, the mechanisms underlying individual cancer susceptibility remain unclear. Here, we took advantage of a unique mouse model of intrinsic developmental heterogeneity (Trim28+/D9) to investigate whether early-life epigenetic variation influences cancer susceptibility later in life. We found that heterozygosity of Trim28 is sufficient to generate two distinct early-life epigenetic states associated with differing cancer susceptibility. These developmentally primed states exhibit differential methylation patterns at typically silenced heterochromatin, detectable as early as 10 days of age. The differentially methylated loci are enriched for genes with known oncogenic potential, frequently mutated in human cancers and correlated with poor prognosis. This study provides genetic evidence that intrinsic developmental heterogeneity can prime individual, lifelong cancer susceptibility.

Single-cell omics: experimental workflow, data analyses and applications

Cells are the fundamental units of biological systems and exhibit unique development trajectories and molecular features. Our exploration of how the genomes orchestrate the formation and maintenance of each cell, and control the cellular phenotypes of various organismsis, is both captivating and intricate. Since the inception of the first single-cell RNA technology, technologies related to single-cell sequencing have experienced rapid advancements in recent years. These technologies have expanded horizontally to include single-cell genome, epigenome, proteome, and metabolome, while vertically, they have progressed to integrate multiple omics data and incorporate additional information such as spatial scRNA-seq and CRISPR screening. Single-cell omics represent a groundbreaking advancement in the biomedical field, offering profound insights into the understanding of complex diseases, including cancers. Here, we comprehensively summarize recent advances in single-cell omics technologies, with a specific focus on the methodology section. This overview aims to guide researchers in selecting appropriate methods for single-cell sequencing and related data analysis.

Multifaceted role of TRIM28 in health and disease

The TRIM (tripartite motif) family, with TRIM28 as a key member, plays a vital role in regulating health and disease. TRIM28 contains various functional domains essential for transcriptional regulation, primarily through its interaction with KRAB-ZNF proteins, which influence chromatin remodeling and gene expression. Despite extensive research, the precise mechanisms by which TRIM28 impacts health and disease remain elusive. This review delves into TRIM28's multifaceted roles in maintaining health, contributing to a variety of diseases, and influencing cancer progression. In cancers, TRIM28 exhibits a dual nature, functioning as both a tumor promoter and suppressor depending on the cellular context and cancer type. The review also explores its critical involvement in processes such as DNA repair, cell cycle regulation, epithelial-to-mesenchymal transition, and the maintenance of stem cell properties. By uncovering TRIM28's complex roles across different cancers and other diseases, this review underscores its potential as a therapeutic target. The significance of TRIM28 as a versatile regulator opens the door to innovative therapeutic strategies, particularly in cancer treatment and the management of other diseases. Ongoing research into TRIM28 may yield key insights into disease progression and novel treatment options.

Clusters of lineage-specific genes are anchored by ZNF274 in repressive perinucleolar compartments

Long known as the site of ribosome biogenesis, the nucleolus is increasingly recognized for its role in shaping three-dimensional (3D) genome organization. Still, the mechanisms governing the targeting of selected regions of the genome to nucleolus-associated domains (NADs) remain enigmatic. Here, we reveal the essential role of ZNF274, a SCAN-bearing member of the Krüppel-associated box (KRAB)-containing zinc finger protein (KZFP) family, in sequestering lineage-specific gene clusters within NADs. Ablation of ZNF274 triggers transcriptional activation across entire genomic neighborhoods-encompassing, among others, protocadherin and KZFP-encoding genes-with loss of repressive chromatin marks, altered the 3D genome architecture and de novo CTCF binding. Mechanistically, ZNF274 anchors target DNA sequences at the nucleolus and facilitates their compartmentalization via a previously uncharted function of the SCAN domain. Our findings illuminate the mechanisms underlying NAD organization and suggest that perinucleolar entrapment into repressive hubs constrains the activation of tandemly arrayed genes to enable selective expression and modulate cell differentiation programs during development.

Pramel15 facilitates zygotic nuclear DNMT1 degradation and DNA demethylation

In mammals, global passive demethylation contributes to epigenetic reprogramming during early embryonic development. At this stage, the majority of DNA-methyltransferase 1 (DNMT1) protein is excluded from nucleus, which is considered the primary cause. However, whether the remaining nuclear activity of DNMT1 is regulated by additional mechanisms is unclear. Here, we report that nuclear DNMT1 abundance is finetuned through proteasomal degradation in mouse zygotes. We identify a maternal factor, Pramel15, which targets DNMT1 for degradation via Cullin-RING E3 ligases. Loss of Pramel15 elevates DNMT1 levels in the zygote pronuclei, impairs zygotic DNA demethylation, and causes a stochastic gain of DNA methylation in early embryos. Thus, Pramel15 can modulate the residual level of DNMT1 in the nucleus during zygotic DNA replication, thereby ensuring efficient DNA methylation reprogramming in early embryos.

Developmental priming of cancer susceptibility

DNA mutations are necessary drivers of cancer, yet only a small subset of mutated cells go on to cause the disease. To date, the mechanisms that determine which rare subset of cells transform and initiate tumorigenesis remain unclear. Here, we take advantage of a unique model of intrinsic developmental heterogeneity (Trim28+/D9) and demonstrate that stochastic early life epigenetic variation can trigger distinct cancer-susceptibility 'states' in adulthood. We show that these developmentally primed states are characterized by differential methylation patterns at typically silenced heterochromatin, and that these epigenetic signatures are detectable as early as 10 days of age. The differentially methylated loci are enriched for genes with known oncogenic potential. These same genes are frequently mutated in human cancers, and their dysregulation correlates with poor prognosis. These results provide proof-of-concept that intrinsic developmental heterogeneity can prime individual, life-long cancer risk.

scTAM-seq enables targeted high-confidence analysis of DNA methylation in single cells

Single-cell DNA methylation profiling currently suffers from excessive noise and/or limited cellular throughput. We developed scTAM-seq, a targeted bisulfite-free method for profiling up to 650 CpGs in up to 10,000 cells per experiment, with a dropout rate as low as 7%. We demonstrate that scTAM-seq can resolve DNA methylation dynamics across B-cell differentiation in blood and bone marrow, identifying intermediate differentiation states that were previously masked. scTAM-seq additionally queries surface-protein expression, thus enabling integration of single-cell DNA methylation information with cell atlas data. In summary, scTAM-seq is a high-throughput, high-confidence method for analyzing DNA methylation at single-CpG resolution across thousands of single cells.

Integrating Micro and Nano Technologies for Cell Engineering and Analysis: Toward the Next Generation of Cell Therapy Workflows

The emerging field of cell therapy offers the potential to treat and even cure a diverse array of diseases for which existing interventions are inadequate. Recent advances in micro and nanotechnology have added a multitude of single cell analysis methods to our research repertoire. At the same time, techniques have been developed for the precise engineering and manipulation of cells. Together, these methods have aided the understanding of disease pathophysiology, helped formulate corrective interventions at the cellular level, and expanded the spectrum of available cell therapeutic options. This review discusses how micro and nanotechnology have catalyzed the development of cell sorting, cellular engineering, and single cell analysis technologies, which have become essential workflow components in developing cell-based therapeutics. The review focuses on the technologies adopted in research studies and explores the opportunities and challenges in combining the various elements of cell engineering and single cell analysis into the next generation of integrated and automated platforms that can accelerate preclinical studies and translational research.

Variable allelic expression of imprinted genes at the Peg13, Trappc9, Ago2 cluster in single neural cells

Genomic imprinting is an epigenetic process through which genes are expressed in a parent-of-origin specific manner resulting in mono-allelic or strongly biased expression of one allele. For some genes, imprinted expression may be tissue-specific and reliant on CTCF-influenced enhancer-promoter interactions. The Peg13 imprinting cluster is associated with neurodevelopmental disorders and comprises canonical imprinted genes, which are conserved between mouse and human, as well as brain-specific imprinted genes in mouse. The latter consist of Trappc9, Chrac1 and Ago2, which have a maternal allelic expression bias of ∼75% in brain. Findings of such allelic expression biases on the tissue level raise the question of how they are reflected in individual cells and whether there is variability and mosaicism in allelic expression between individual cells of the tissue. Here we show that Trappc9 and Ago2 are not imprinted in hippocampus-derived neural stem cells (neurospheres), while Peg13 retains its strong bias of paternal allele expression. Upon analysis of single neural stem cells and in vitro differentiated neurons, we find not uniform, but variable states of allelic expression, especially for Trappc9 and Ago2. These ranged from mono-allelic paternal to equal bi-allelic to mono-allelic maternal, including biased bi-allelic transcriptional states. Even Peg13 expression deviated from its expected paternal allele bias in a small number of cells. Although the cell populations consisted of a mosaic of cells with different allelic expression states, as a whole they reflected bulk tissue data. Furthermore, in an attempt to identify potential brain-specific regulatory elements across the Trappc9 locus, we demonstrate tissue-specific and general silencer activities, which might contribute to the regulation of its imprinted expression bias.

Maternal genetic factors in the development of congenital heart defects

Congenital heart defects (CHDs) are among the most common, serious birth defects. However, the cause of CHDs is unknown for approximately half of affected individuals and there are few prevention strategies. Although not extensively investigated, maternal genes may contribute to CHD etiology by modifying the effects of maternal exposures (e.g. medications, nutrients), contributing to maternal phenotypes that are associated with an increased risk of CHDs in offspring (e.g. diabetes), or acting as maternal effect genes. Since maternal genes could serve as a target for the primary prevention of CHDs, efforts to further define the contribution of the maternal genome to CHD etiology are warranted.

Epigenetic Regulation in Knee Osteoarthritis

Osteoarthritis (OA) is a complicated disease with both hereditary and environmental causes. Despite an increase in reports of possible OA risk loci, it has become clear that genetics is not the sole cause of osteoarthritis. Epigenetics, which can be triggered by environmental influences and result in transcriptional alterations, may have a role in OA pathogenesis. The majority of recent research on the epigenetics of OA has been focused on DNA methylation, histone modification, and non-coding RNAs. However, this study will explore epigenetic regulation in OA at the present stage. How genetics, environmental variables, and epigenetics interact will be researched, shedding light for future studies. Their possible interaction and control processes open up new avenues for the development of innovative osteoarthritis treatment and diagnostic techniques.

Chemoenzymatic labeling of DNA methylation patterns for single-molecule epigenetic mapping

DNA methylation, specifically, methylation of cytosine (C) nucleotides at the 5-carbon position (5-mC), is the most studied and significant epigenetic modification. Here we developed a chemoenzymatic procedure to fluorescently label non-methylated cytosines in CpG context, allowing epigenetic profiling of single DNA molecules spanning hundreds of thousands of base pairs. We used a CpG methyltransferase with a synthetic S-adenosyl-l-methionine cofactor analog to transfer an azide to cytosines instead of the natural methyl group. A fluorophore was then clicked onto the DNA, reporting on the amount and position of non-methylated CpGs. We found that labeling efficiency was increased up to 2-fold by the addition of a nucleosidase, presumably by degrading the inactive by-product of the cofactor after labeling, preventing its inhibitory effect. We used the method to determine the decline in global DNA methylation in a chronic lymphocytic leukemia patient and then performed whole-genome methylation mapping of the model plant Arabidopsis thaliana. Our genome maps show high concordance with published bisulfite sequencing methylation maps. Although mapping resolution is limited by optical detection to 500–1000 bp, the labeled DNA molecules produced by this approach are hundreds of thousands of base pairs long, allowing access to long repetitive and structurally variable genomic regions.

scTEM-seq: Single-cell analysis of transposable element methylation to link global epigenetic heterogeneity with transcriptional programs

Global changes in DNA methylation are observed in development and disease, and single-cell analyses are highlighting the heterogeneous regulation of these processes. However, technical challenges associated with single-cell analysis of DNA methylation limit these studies. We present single-cell transposable element methylation sequencing (scTEM-seq) for cost-effective estimation of average DNA methylation levels. By targeting high-copy SINE Alu elements, we achieve amplicon bisulphite sequencing with thousands of loci covered in each scTEM-seq library. Parallel transcriptome analysis is also performed to link global DNA methylation estimates with gene expression. We apply scTEM-seq to KG1a acute myeloid leukaemia (AML) cells, and primary AML cells. Our method reveals global DNA methylation heterogeneity induced by decitabine treatment of KG1a cells associated with altered expression of immune process genes. We also compare global DNA methylation estimates to expression of transposable elements and find a predominance of negative correlations. Finally, we observe co-ordinated upregulation of many transposable elements in a sub-set of decitabine treated cells. By linking global DNA methylation heterogeneity with transcription, scTEM-seq will refine our understanding of epigenetic regulation in cancer and beyond.

Maternal effect genes: Update and review of evidence for a link with birth defects

Maternal effect genes (MEGs) encode factors (e.g., RNA) that are present in the oocyte and required for early embryonic development. Hence, while these genes and gene products are of maternal origin, their phenotypic consequences result from effects on the embryo. The first mammalian MEGs were identified in the mouse in 2000 and were associated with early embryonic loss in the offspring of homozygous null females. In humans, the first MEG was identified in 2006, in women who had experienced a range of adverse reproductive outcomes, including hydatidiform moles, spontaneous abortions, and stillbirths. Over 80 mammalian MEGs have subsequently been identified, including several that have been associated with phenotypes in humans. In general, pathogenic variants in MEGs or the absence of MEG products are associated with a spectrum of adverse outcomes, which in humans range from zygotic cleavage failure to offspring with multi-locus imprinting disorders. Although less established, there is also evidence that MEGs are associated with structural birth defects (e.g., craniofacial malformations, congenital heart defects). This review provides an updated summary of mammalian MEGs reported in the literature through early 2021, as well as an overview of the evidence for a link between MEGs and structural birth defects.

Genetic Studies on Mammalian DNA Methyltransferases

Cytosine methylation at the C5-position-generating 5-methylcytosine (5mC)-is a DNA modification found in many eukaryotic organisms, including fungi, plants, invertebrates, and vertebrates, albeit its levels vary greatly in different organisms. In mammals, cytosine methylation occurs predominantly in the context of CpG dinucleotides, with the majority (60-80%) of CpG sites in their genomes being methylated. DNA methylation plays crucial roles in the regulation of chromatin structure and gene expression and is essential for mammalian development. Aberrant changes in DNA methylation and genetic alterations in enzymes and regulators involved in DNA methylation are associated with various human diseases, including cancer and developmental disorders. In mammals, DNA methylation is mediated by two families of DNA methyltransferases (Dnmts), namely Dnmt1 and Dnmt3 proteins. Over the last three decades, genetic manipulations of these enzymes, as well as their regulators, in mice have greatly contributed to our understanding of the biological functions of DNA methylation in mammals. In this chapter, we discuss genetic studies on mammalian Dnmts, focusing on their roles in embryogenesis, cellular differentiation, genomic imprinting, and human diseases.

Single-cell CUT&Tag profiles histone modifications and transcription factors in complex tissues

In contrast to single-cell approaches for measuring gene expression and DNA accessibility, single-cell methods for analyzing histone modifications are limited by low sensitivity and throughput. Here, we combine the CUT&Tag technology, developed to measure bulk histone modifications, with droplet-based single-cell library preparation to produce high-quality single-cell data on chromatin modifications. We apply single-cell CUT&Tag (scCUT&Tag) to tens of thousands of cells of the mouse central nervous system and probe histone modifications characteristic of active promoters, enhancers and gene bodies (H3K4me3, H3K27ac and H3K36me3) and inactive regions (H3K27me3). These scCUT&Tag profiles were sufficient to determine cell identity and deconvolute regulatory principles such as promoter bivalency, spreading of H3K4me3 and promoter-enhancer connectivity. We also used scCUT&Tag to investigate the single-cell chromatin occupancy of transcription factor OLIG2 and the cohesin complex component RAD21. Our results indicate that analysis of histone modifications and transcription factor occupancy at single-cell resolution provides unique insights into epigenomic landscapes in the central nervous system.

DNA methylation studies in cattle

Investigation of the role of epigenetics in cattle breeding is gaining importance. DNA methylation represents an epigenetic modification which is essential for genomic stability and maintenance of development. Recently, DNA methylation research in cattle has intensified. The studies focus on the definition of methylomes in various organs and tissues in relation to the expression of genes underlying economically important traits, and explore methylome changes under developmental, environmental, disease, and diet influences. The investigations further characterize the methylation patterns of gametes in connection with their quality, and study methylome alterations in the developing naturally or assisted produced zygotes, embryos, and fetuses, considering their viability. A wide array of technologies developed for accurate and precise analysis of DNA methylation patterns is employed for both single-gene and genome-wide studies. Overall, the research is directed towards the identification of single methylation markers or their combinations which may be useful in the selection and breeding of animals to ensure cattle improvement.

Prospects for Use of Single-Cell Sequencing to Assess DNA Methylation in Asthma

Asthma is a complex disease with an increasing prevalence rate caused by the interaction of multiple genetically inherited and environmental factors. Epigenetics link genetic susceptibility and environmental factors. DNA methylation is an epigenetic modification that plays a crucial role in the regulation of growth and development, gene expression, and disease. Relatively little is known about DNA methylation in asthma, with few studies to date using single-cell sequencing to analyze the molecular mechanism by which DNA methylation regulates asthma. Cells with similar phenotypes may be heterogeneous in function and transcription, as may their genetic information. Although multi-omics methods, such as studies of the genome, transcriptome, and epigenome, can be used to evaluate biological processes, these methods are applicable only to groups of cells or tissues and provide averages that may obscure direct correlations among multiple layers of data. Single-cell sequencing technology can clarify the methylation and expression of genes in different populations of cells, in contrast to traditional multi-omics sequencing, which can determine only average values of cell populations. Single-cell sequence can therefore better reflect the pathogenesis of asthma, as it can clarify the function and regulatory mechanism of DNA methylation in asthma, and detect new genes and molecular markers that may become therapeutic targets in this disease.

Parallel bimodal single-cell sequencing of transcriptome and chromatin accessibility

Joint profiling of transcriptome and chromatin accessibility within single cells allows for the deconstruction of the complex relationship between transcriptional states and upstream regulatory programs determining different cell fates. Here, we developed an automated method with high sensitivity, assay for single-cell transcriptome and accessibility regions (ASTAR-seq), for simultaneous measurement of whole-cell transcriptome and chromatin accessibility within the same single cell. To show the utility of ASTAR-seq, we profiled 384 mESCs under naive and primed pluripotent states as well as a two-cell like state, 424 human cells of various lineage origins (BJ, K562, JK1, and Jurkat), and 480 primary cord blood cells undergoing erythroblast differentiation. With the joint profiles, we configured the transcriptional and chromatin accessibility landscapes of discrete cell states, uncovered linked sets of cis-regulatory elements and target genes unique to each state, and constructed interactome and transcription factor (TF)–centered upstream regulatory networks for various cell states.

Single-Cell Multi-Omics and Its Prospective Application in Cancer Biology

In recent years, the emergence of single-cell omics technologies, which can profile genomics, transcriptomics, epigenomics, and proteomics, has provided unprecedented insights into characteristics of cancer, enabling higher resolution and accuracy to decipher the cellular and molecular mechanisms relating to tumorigenesis, evolution, metastasis, and immune responses. Single-cell multi-omics technologies, which are developed based on the combination of multiple single-cell mono-omics technologies, can simultaneously analyze RNA expression, single nucleotide polymorphism, epigenetic modification, or protein abundance, enabling the in-depth understanding of gene expression regulatory mechanisms. In this review, the state-of-the-art single-cell multi-omics technologies are summarized and the prospects of their application in cancer biology are discussed.

Infertility-Causing Haploinsufficiency Reveals TRIM28/KAP1 Requirement in Spermatogonia

Spermatogenesis relies on exquisite stem cell homeostasis, the carefully balanced self-renewal and differentiation of spermatogonial stem cells (SSCs). Disturbing this equilibrium will likely manifest through sub- or infertility, a global health issue with often idiopathic presentation. In this respect, disease phenotypes caused by haploinsufficiency of otherwise vital developmental genes are of particular interest. Here, we show that mice heterozygous for Trim28, an essential epigenetic regulator, suffer gradual testicular degeneration. Contrary to previous reports we detect Trim28 expression in spermatogonia, albeit at low levels. Further reduction through Trim28 heterozygosity increases the propensity of SSCs to differentiate at the cost of self-renewal.

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TRIM28/KAP1 haploinsufficiency causes testicular degeneration and infertility

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TRIM28/KAP1 is expressed in spermatogonia stem cell compartment

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Stem cell homeostasis in the testis is dependent on proper TRIM28/KAP1 levels

Microfluidic Single-Cell Omics Analysis

The commonly existing cellular heterogeneity plays a critical role in biological processes such as embryonic development, cell differentiation, and disease progress. Single-cell omics-based heterogeneous studies have great significance for identifying different cell populations, discovering new cell types, revealing informative cell features, and uncovering significant interrelationships between cells. Recently, microfluidics has evolved to be a powerful technology for single-cell omics analysis due to its merits of throughput, sensitivity, and accuracy. Herein, the recent advances of microfluidic single-cell omics analysis, including different microfluidic platform designs, lysis strategies, and omics analysis techniques, are reviewed. Representative applications of microfluidic single-cell omics analysis in complex biological studies are then summarized. Finally, a few perspectives on the future challenges and development trends of microfluidic-assisted single-cell omics analysis are discussed.

Microfluidic epigenomic mapping technologies for precision medicine

Epigenomic mapping of tissue samples generates critical insights into genome-wide regulations of gene activities and expressions during normal development and disease processes. Epigenomic profiling using a low number of cells produced by patient and mouse samples presents new challenges to biotechnologists. In this review, we first discuss the rationale and premise behind profiling epigenomes for precision medicine. We then examine the existing literature on applying microfluidics to facilitate low-input and high-throughput epigenomic profiling, with emphasis on technologies enabling interfacing with next-generation sequencing. We detail assays on studies of histone modifications, DNA methylation, 3D chromatin structures and non-coding RNAs. Finally, we discuss what the future may hold in terms of method development and translational potential.

Personalized Epigenome Remodeling Under Biochemical and Psychological Changes During Long-Term Isolation Environment

It has been reported that several aspects of human health could be disturbed during a long-term isolated environment (for instance, the Mars-500 mission), including psychiatric disorders, circadian disruption, temporal dynamics of gut microbiota, immune responses, and physical-activity-related neuromuscular performance. Nevertheless, the mechanisms underlying these disturbances and the interactions among different aspects of human adaptation to extreme environments remain to be elucidated. Epigenetic features, like DNA methylation, might be a linking mechanism that explains the involvement of environmental factors between the human genome and the outcome of health. We conducted an exploration of personalized longitudinal DNA methylation patterns of the peripheral whole blood cells, profiling six subjects across six sampling points in the Mars-500 mission. Specifically, we developed a Personalized Epigenetic-Phenotype Synchronization Analysis (PeSa) algorithm to explore glucose- and mood-state-synchronized DNA methylation sites, focusing on finding the dynamic associations between epigenetic patterns and phenotypes in each individual, and exploring the underling epigenetic connections between glucose and mood-state disturbance. Results showed that DMPs (differentially methylated-probes) were significantly enriched in pathways related to glucose metabolism (Type II diabetes mellitus pathway), mood state (Long-term depression) and circadian rhythm (Circadian entrainment pathway) during the mission. Furthermore, our data revealed individualized glucose-synchronized and mood-state-synchronized DNA methylation sites, and PTPRN2 was found to be associated with both glucose and mood state disturbances across all six subjects. Our findings suggest that personalized phenotype-synchronized epigenetic features could reflect the effects on the human body, including the disturbances of glucose and mood-states. The association analysis of DNA methylation and phenotypes, like the PeSa analysis, could provide new possibilities in understanding the intrinsic relationship between phenotypic changes of the human body adapting to long-term isolation environmental factors.

The KRAB-zinc-finger protein ZFP708 mediates epigenetic repression at RMER19B retrotransposons

Global epigenetic reprogramming is vital to purge germ cell-specific epigenetic features to establish the totipotent state of the embryo. This process transpires to be carefully regulated and is not an undirected, radical erasure of parental epigenomes. The TRIM28 complex has been shown to be crucial in embryonic epigenetic reprogramming by regionally opposing DNA demethylation to preserve vital parental information to be inherited from germline to soma. Yet the DNA-binding factors guiding this complex to specific targets are largely unknown. Here, we uncover and characterize a novel, maternally expressed, TRIM28-interacting KRAB zinc-finger protein: ZFP708. It recruits the repressive TRIM28 complex to RMER19B retrotransposons to evoke regional heterochromatin formation. ZFP708 binding to these hitherto unknown TRIM28 targets is DNA methylation and H3K9me3 independent. ZFP708 mutant mice are viable and fertile, yet embryos fail to inherit and maintain DNA methylation at ZFP708 target sites. This can result in activation of RMER19B-adjacent genes, while ectopic expression of ZFP708 results in transcriptional repression. Finally, we describe the evolutionary conservation of ZFP708 in mice and rats, which is linked to the conserved presence of the targeted RMER19B retrotransposons in these species.

Summary: Analysis of the function and targets of a maternal KRAB-zinc-finger protein, ZFP708, found to specifically mediate maintenance of DNA methylation at a subset of LTR retrotransposons during embryonic epigenetic reprogramming.

Mapping the Mouse Cell Atlas by Microwell-Seq

Single-cell RNA sequencing (scRNA-seq) technologies are poised to reshape the current cell-type classification system. However, a transcriptome-based single-cell atlas has not been achieved for complex mammalian systems. Here, we developed Microwell-seq, a high-throughput and low-cost scRNA-seq platform using simple, inexpensive devices. Using Microwell-seq, we analyzed more than 400,000 single cells covering all of the major mouse organs and constructed a basic scheme for a mouse cell atlas (MCA). We reveal a single-cell hierarchy for many tissues that have not been well characterized previously. We built a web-based "single-cell MCA analysis" pipeline that accurately defines cell types based on single-cell digital expression. Our study demonstrates the wide applicability of the Microwell-seq technology and MCA resource.

Single-cell microRNA-mRNA co-sequencing reveals non-genetic heterogeneity and mechanisms of microRNA regulation

Measuring multiple omics profiles from the same single cell opens up the opportunity to decode molecular regulation that underlies intercellular heterogeneity in development and disease. Here, we present co-sequencing of microRNAs and mRNAs in the same single cell using a half-cell genomics approach. This method demonstrates good robustness (~95% success rate) and reproducibility (R2 = 0.93 for both microRNAs and mRNAs), yielding paired half-cell microRNA and mRNA profiles, which we can independently validate. By linking the level of microRNAs to the expression of predicted target mRNAs across 19 single cells that are phenotypically identical, we observe that the predicted targets are significantly anti-correlated with the variation of abundantly expressed microRNAs. This suggests that microRNA expression variability alone may lead to non-genetic cell-to-cell heterogeneity. Genome-scale analysis of paired microRNA-mRNA co-profiles further allows us to derive and validate regulatory relationships of cellular pathways controlling microRNA expression and intercellular variability.

Analysis of genome-wide in cell free DNA methylation: progress and prospect

In this work, we focus on the detection methods of cfDNA methylation based on NGS and the latest progress.

Microfluidic and Paper-Based Devices for Disease Detection and Diagnostic Research

Recent developments in microfluidic devices, nanoparticle chemistry, fluorescent microscopy, and biochemical techniques such as genetic identification and antibody capture have provided easier and more sensitive platforms for detecting and diagnosing diseases as well as providing new fundamental insight into disease progression. These advancements have led to the development of new technology and assays capable of easy and early detection of pathogenicity as well as the enhancement of the drug discovery and development pipeline. While some studies have focused on treatment, many of these technologies have found initial success in laboratories as a precursor for clinical applications. This review highlights the current and future progress of microfluidic techniques geared toward the timely and inexpensive diagnosis of disease including technologies aimed at high-throughput single cell analysis for drug development. It also summarizes novel microfluidic approaches to characterize fundamental cellular behavior and heterogeneity.

Epigenetic control of gene regulation during development and disease: A view from the retina

Complex biological processes, such as organogenesis and homeostasis, are stringently regulated by genetic programs that are fine-tuned by epigenetic factors to establish cell fates and/or to respond to the microenvironment. Gene regulatory networks that guide cell differentiation and function are modulated and stabilized by modifications to DNA, RNA and proteins. In this review, we focus on two key epigenetic changes - DNA methylation and histone modifications - and discuss their contribution to retinal development, aging and disease, especially in the context of age-related macular degeneration (AMD) and diabetic retinopathy. We highlight less-studied roles of DNA methylation and provide the RNA expression profiles of epigenetic enzymes in human and mouse retina in comparison to other tissues. We also review computational tools and emergent technologies to profile, analyze and integrate epigenetic information. We suggest implementation of editing tools and single-cell technologies to trace and perturb the epigenome for delineating its role in transcriptional regulation. Finally, we present our thoughts on exciting avenues for exploring epigenome in retinal metabolism, disease modeling, and regeneration.

DNA methylation dynamics of genomic imprinting in mouse development

DNA methylation is an essential epigenetic mark crucial for normal mammalian development. This modification controls the expression of a unique class of genes, designated as imprinted, which are expressed monoallelically and in a parent-of-origin-specific manner. Proper parental allele-specific DNA methylation at imprinting control regions (ICRs) is necessary for appropriate imprinting. Processes that deregulate DNA methylation of imprinted loci cause disease in humans. DNA methylation patterns dramatically change during mammalian development: first, the majority of the genome, with the exception of ICRs, is demethylated after fertilization, and subsequently undergoes genome-wide de novo DNA methylation. Secondly, after primordial germ cells are specified in the embryo, another wave of demethylation occurs, with ICR demethylation occurring late in the process. Lastly, ICRs reacquire DNA methylation imprints in developing germ cells. We describe the past discoveries and current literature defining these crucial dynamics in relation to imprinted genes and the rest of the genome.

The Human Cell Atlas: Technical approaches and challenges

The Human Cell Atlas is a large, international consortium that aims to identify and describe every cell type in the human body. The comprehensive cellular maps that arise from this ambitious effort have the potential to transform many aspects of fundamental biology and clinical practice. Here, we discuss the technical approaches that could be used today to generate such a resource and also the technical challenges that will be encountered.

Single-Cell (Multi)omics Technologies

Single-cell multiomics technologies typically measure multiple types of molecule from the same individual cell, enabling more profound biological insight than can be inferred by analyzing each molecular layer from separate cells. These single-cell multiomics technologies can reveal cellular heterogeneity at multiple molecular layers within a population of cells and reveal how this variation is coupled or uncoupled between the captured omic layers. The data sets generated by these techniques have the potential to enable a deeper understanding of the key biological processes and mechanisms driving cellular heterogeneity and how they are linked with normal development and aging as well as disease etiology. This review details both established and novel single-cell mono- and multiomics technologies and considers their limitations, applications, and likely future developments.

Single-Cell DNA Methylation Profiling: Technologies and Biological Applications

DNA methylation is an epigenetic modification that plays an important role in gene expression regulation, development, and disease. Recent technological innovations have spurred the development of methods that enable us to study the occurrence and biology of this mark at the single-cell level. Apart from answering fundamental biological questions about heterogeneous systems or rare cell types, low-input methods also bring clinical applications within reach. Ultimately, integrating these data with other single-cell data sets will allow deciphering multiple layers of gene expression regulation within each individual cell. Here, we review the approaches that have been developed to facilitate single-cell DNA methylation profiling, their biological applications, and how these will further our understanding of the biology of DNA methylation.

Individual retrotransposon integrants are differentially controlled by KZFP/KAP1-dependent histone methylation, DNA methylation and TET-mediated hydroxymethylation in naïve embryonic stem cells

Background

The KZFP/KAP1 (KRAB zinc finger proteins/KRAB-associated protein 1) system plays a central role in repressing transposable elements (TEs) and maintaining parent-of-origin DNA methylation at imprinting control regions (ICRs) during the wave of genome-wide reprogramming that precedes implantation. In naïve murine embryonic stem cells (mESCs), the genome is maintained highly hypomethylated by a combination of TET-mediated active demethylation and lack of de novo methylation, yet KAP1 is tethered by sequence-specific KZFPs to ICRs and TEs where it recruits histone and DNA methyltransferases to impose heterochromatin formation and DNA methylation.

Results

Here, upon removing either KAP1 or the cognate KZFP, we observed rapid TET2-dependent accumulation of 5hmC at both ICRs and TEs. In the absence of the KZFP/KAP1 complex, ICRs lost heterochromatic histone marks and underwent both active and passive DNA demethylation. For KAP1-bound TEs, 5mC hydroxylation correlated with transcriptional reactivation. Using RNA-seq, we further compared the expression profiles of TEs upon Kap1 removal in wild-type, Dnmt and Tet triple knockout mESCs. While we found that KAP1 represents the main effector of TEs repression in all three settings, we could additionally identify specific groups of TEs further controlled by DNA methylation. Furthermore, we observed that in the absence of TET proteins, activation upon Kap1 depletion was blunted for some TE integrants and increased for others.

Conclusions

Our results indicate that the KZFP/KAP1 complex maintains heterochromatin and DNA methylation at ICRs and TEs in naïve embryonic stem cells partly by protecting these loci from TET-mediated demethylation. Our study further unveils an unsuspected level of complexity in the transcriptional control of the endovirome by demonstrating often integrant-specific differential influences of histone-based heterochromatin modifications, DNA methylation and 5mC oxidation in regulating TEs expression.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0177-1) contains supplementary material, which is available to authorized users.

A roadmap towards personalized immunology

Big data generation and computational processing will enable medicine to evolve from a "one-size-fits-all" approach to precise patient stratification and treatment. Significant achievements using "Omics" data have been made especially in personalized oncology. However, immune cells relative to tumor cells show a much higher degree of complexity in heterogeneity, dynamics, memory-capability, plasticity and "social" interactions. There is still a long way ahead on translating our capability to identify potentially targetable personalized biomarkers into effective personalized therapy in immune-centralized diseases. Here, we discuss the recent advances and successful applications in "Omics" data utilization and network analysis on patients' samples of clinical trials and studies, as well as the major challenges and strategies towards personalized stratification and treatment for infectious or non-communicable inflammatory diseases such as autoimmune diseases or allergies. We provide a roadmap and highlight experimental, clinical, computational analysis, data management, ethical and regulatory issues to accelerate the implementation of personalized immunology.

Mosaicism for GNAS methylation defects associated with pseudohypoparathyroidism type 1B arose in early post-zygotic phases

Background

Pseudohypoparathyroidism type 1B (PHP1B; MIM#603233) is a rare imprinting disorder (ID), associated with the GNAS locus, characterized by parathyroid hormone (PTH) resistance in the absence of other endocrine or physical abnormalities. Sporadic PHP1B cases, with no known underlying primary genetic lesions, could represent true stochastic errors in early embryonic maintenance of methylation. Previous data confirmed the existence of different degrees of methylation defects associated with PHP1B and suggested the presence of mosaicism, a phenomenon already described in the context of other IDs.

Results

With respect to mosaic conditions, the study of multiple tissues is a necessary approach; thus, we investigated somatic cell lines (peripheral blood and buccal epithelium and cells from the urine sediment) descending from different germ layers from 19 PHP patients (11 spor-PHP1B, 4 GNAS mutated PHP1A, and 4 PHP with no GNAS (epi)genetic defects) and 5 healthy controls. We identified 11 patients with epigenetic defects, further subdivided in groups with complete or partial methylation defects. The recurrence of specific patterns of partial methylation defects limited to specific CpGs was confirmed by checking methylation profiles of spor-PHP1B patients diagnosed in our lab (n = 56). Underlying primary genetic defects, such as uniparental disomy or deletion, potentially causative for the detected partial methylation were excluded in all samples.

Conclusions

Our data showed no differences of methylation levels between organs and tissues from the same patient, so we concluded that the epimutation occurred in early post-zygotic phases and that the partial defects were mosaics. The number of patients with no detectable (epi)genetic GNAS defects was too small to exclude epimutations occurring in later post-zygotic phases, affecting only selected tissues different from blood, thus leading to underdiagnosis during routine molecular diagnosis. Finally, we found no correlation between methylation ratios, representing the proportion of epimutated cells, and the clinical presentation, further confirming the hypothesis of a threshold effect of the GNAS loss of imprinting leading to an "all-or-none" phenotype.

Novel reproductive technologies to prevent mitochondrial disease

BACKGROUND

The use of nuclear transfer (NT) has been proposed as a novel reproductive treatment to overcome the transmission of maternally-inherited mitochondrial DNA (mtDNA) mutations. Pathogenic mutations in mtDNA can cause a wide-spectrum of life-limiting disorders, collectively known as mtDNA disease, for which there are currently few effective treatments and no known cures. The many unique features of mtDNA make genetic counselling challenging for women harbouring pathogenic mtDNA mutations but reproductive options that involve medical intervention are available that will minimize the risk of mtDNA disease in their offspring. This includes PGD, which is currently offered as a clinical treatment but will not be suitable for all. The potential for NT to reduce transmission of mtDNA mutations has been demonstrated in both animal and human models, and has recently been clinically applied not only to prevent mtDNA disease but also for some infertility cases. In this review, we will interrogate the different NT techniques, including a discussion on the available safety and efficacy data of these technologies for mtDNA disease prevention. In addition, we appraise the evidence for the translational use of NT technologies in infertility.

OBJECTIVE AND RATIONALE

We propose to review the current scientific evidence regarding the clinical use of NT to prevent mitochondrial disease.

SEARCH METHODS

The scientific literature was investigated by searching PubMed database until Jan 2017. Relevant documents from Human Fertilisation and Embryology Authority as well as reports from both the scientific and popular media were also implemented. The above searches were based on the following key words: 'mitochondria', 'mitochondrial DNA'; 'mitochondrial DNA disease', 'fertility'; 'preimplantation genetic diagnosis', 'nuclear transfer', 'mitochondrial replacement' and 'mitochondrial donation'.

OUTCOMES

While NT techniques have been shown to effectively reduce the transmission of heteroplasmic mtDNA variants in animal models, and increasing evidence supports their use to prevent the transmission of human mtDNA disease, the need for robust, long-term evaluation is still warranted. Moreover, prenatal screening would still be strongly advocated in combination with the use of these IVF-based technologies. Scientific evidence to support the use of NT and other novel reproductive techniques for infertility is currently lacking.

WIDER IMPLICATIONS

It is mandatory that any new ART treatments are first adequately assessed in both animal and human models before the cautious implementation of these new therapeutic approaches is clinically undertaken. There is growing evidence to suggest that the translation of these innovative technologies into clinical practice should be cautiously adopted only in highly selected patients. Indeed, given the limited safety and efficacy data, close monitoring of any offspring remains paramount.

ZFP57 and the Targeted Maintenance of Postfertilization Genomic Imprints

Epigenetic modifications play an important role in modulating genome function. In mammals, inappropriate epigenetic states can cause embryonic lethality and various acquired and inherited diseases; hence, it is important to understand how such states are formed and maintained in particular genomic contexts. Genomic imprinting is a process in which epigenetic states provide a sustained memory of parental origin and cause gene expression/repression from only one of the two parental chromosomes. Genomic imprinting is therefore a valuable model to decipher the principles and processes associated with the targeting and maintenance of epigenetic states in general. Krüppel-associated box zinc finger proteins (KRAB-ZFPs) are proteins that have the potential to mediate this. ZFP57, one of the best characterized proteins in this family, has been shown to target and maintain epigenetic states at imprinting control regions after fertilization. Its role in imprinting through the use of ZFP57 mutants in mouse and the wider implications of KRAB-ZFPs for the targeted maintenance of epigenetic states are discussed here.

The Human Cell Atlas

The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

The Human Cell Atlas

The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

From Germline to Soma: Epigenetic Dynamics in the Mouse Preimplantation Embryo

When reflecting about cell fate commitment we think of differentiation. Be it during embryonic development or in an adult stem cell niche, where cells of a higher potency specialize and cell fate decisions are taken. Under normal circumstances this process is definitive and irreversible. Cell fate commitment is achieved by the establishment of cell-type-specific transcriptional programmes, which in turn are guided, reinforced, and ultimately locked-in by epigenetic mechanisms. Yet, this plunging drift in cellular potency linked to epigenetically restricted access to genomic information is problematic for reproduction. Particularly in mammals where germ cells are not set aside early on like in other species. Instead they are rederived from the embryonic ectoderm, a differentiating embryonic tissue with somatic epigenetic features. The epigenomes of germ cell precursors are efficiently reprogrammed against the differentiation trend, only to specialize once more into highly differentiated, sex-specific gametes: oocyte and sperm. Their differentiation state is reflected in their specialized epigenomes, and erasure of these features is required to enable the acquisition of the totipotent cell fate to kick start embryonic development of the next generation. Recent technological advances have enabled unprecedented insights into the epigenetic dynamics, first of DNA methylation and then of histone modifications, greatly expanding the historically technically limited understanding of this processes. In this chapter we will focus on the details of embryonic epigenetic reprogramming, a cell fate determination process against the tide to a higher potency.

Age-related clonal hematopoiesis

Age-related alterations in the human blood system occur in B cells, T cells, cells of the innate system, as well as hematopoietic stem and progenitor cells (HSPCs). Interestingly, age-related, reduced genetic diversity can be identified at the stem cell level and also independently in B cells and T cells. This reduced diversity is most probably related to somatic mutations or to changes in the microenvironmental niche. Either process can select for specific clones or cause repeated evolutionary bottlenecks. This review discusses the age-related clonal expansions in the human HSPC pool, which was termed in the past age-related clonal hematopoiesis (ARCH). ARCH is defined as the gradual, clonal expansion of HSPCs carrying specific, disruptive, and recurrent genetic variants, in individuals without clear diagnosis of hematological malignancies. ARCH is associated not just with chronological aging but also with several other, age-related pathological conditions, including inflammation, vascular diseases, cancer mortality, and high risk for hematological malignancies. Although it remains unclear whether ARCH is a marker of aging or plays an active role in these various pathophysiologies, it is suggested here that treating or even preventing ARCH may prove to be beneficial for human health. This review also describes a decision tree for the diagnosis and follow-up for ARCH in a research setting.

Human embryos from induced pluripotent stem cell-derived gametes: ethical and quality considerations

Protocols for successful differentiation of male and female gametes from induced pluripotent stem cells have been published. Although culture of precursor cells in a natural microenvironment remains necessary to achieve terminal differentiation, the creation of human preimplantation embryos from induced pluripotent stem cell-derived gametes is technically feasible. Such embryos could provide a solution to the scarcity of human cleavage-stage embryos donated for research. Here, we discuss current technology, major research-related ethical concerns and propose the norms that would assure the quality and reliability of such embryos.

A short review of variants calling for single-cell-sequencing data with applications

The field of single-cell sequencing is fleetly expanding, and many techniques have been developed in the past decade. With this technology, biologists can study not only the heterogeneity between two adjacent cells in the same tissue or organ, but also the evolutionary relationships and degenerative processes in a single cell. Calling variants is the main purpose in analyzing single cell sequencing (SCS) data. Currently, some popular methods used for bulk-cell-sequencing data analysis are tailored directly to be applied in dealing with SCS data. However, SCS requires an extra step of genome amplification to accumulate enough quantity for satisfying sequencing needs. The amplification yields large biases and thus raises challenge for using the bulk-cell-sequencing methods. In order to provide guidance for the development of specialized analyzed methods as well as using currently developed tools for SNS, this paper aims to bridge the gap. In this paper, we firstly introduced two popular genome amplification methods and compared their capabilities. Then we introduced a few popular models for calling single-nucleotide polymorphisms and copy-number variations. Finally, break-through applications of SNS were summarized to demonstrate its potential in researching cell evolution.

TRIM28 regulates Igf2-H19 and Dlk1-Gtl2 imprinting by distinct mechanisms during sheep fibroblast proliferation

DNA methylation is an essential epigenetic modification involved in regulating gene expression and maintaining epigenetic information across generations. However, how these marks are recognized and interpreted to activate or repress imprinted genes is not fully understood. Preliminary evidence describes the transcriptional repressor TRIM28 as a key regulator of imprinted gene expression during and after early genome-wide reprogramming. Aberrant expression of imprinted genes maybe one possible cause of incomplete epigenetic reprogramming and low efficiency in somatic cell nuclear transfer. Here, we perform a series of experiments to determine whether knockdown of Trim28 alters imprinted gene expression and DMR methylation in sheep embryonic fibroblast (SEF) cells. siRNA-mediated Trim28 silencing in SEF cells resulted in significantly decreased expression of Gtl2 to 30% and increased expression of Dlk1 (~1.7-fold). Moreover, knocking down Trim28 induced DNA methylation at the IG-DMR and the Gtl2 promoter was disrupted. Here, we uncover an important role for Trim28 in the maintenance of DNA methylation at IG-DMR during replication-dependent dilution of methylated cytosine during cellular proliferation. Unlike Dlk1-Gtl2 however, knocking down Trim28 does not affect DMR methylation in the Igf2-H19 gene cluster, yet results in increased expression of Igf2 and H19. Interestingly, Peg3 expression decreased by 60% in Trim28 knockdown cells. PEG3 as a transcriptional repressor to the H19-ICR that interacts with the co-repressor protein TRIM28 through KRAB-A. Trim28 therefore appears to control the Igf2-H19 imprinted cluster indirectly via PEG3, which is distinct from its classical role in preserving DNA methylation during DNA replication. Our results therefore indicate that Trim28 regulates imprinted gene expression through at least two distinct mechanisms during cells proliferation.

Bisulfite-independent analysis of CpG island methylation enables genome-scale stratification of single cells

Conventional DNA bisulfite sequencing has been extended to single cell level, but the coverage consistency is insufficient for parallel comparison. Here we report a novel method for genome-wide CpG island (CGI) methylation sequencing for single cells (scCGI-seq), combining methylation-sensitive restriction enzyme digestion and multiple displacement amplification for selective detection of methylated CGIs. We applied this method to analyzing single cells from two types of hematopoietic cells, K562 and GM12878 and small populations of fibroblasts and induced pluripotent stem cells. The method detected 21 798 CGIs (76% of all CGIs) per cell, and the number of CGIs consistently detected from all 16 profiled single cells was 20 864 (72.7%), with 12 961 promoters covered. This coverage represents a substantial improvement over results obtained using single cell reduced representation bisulfite sequencing, with a 66-fold increase in the fraction of consistently profiled CGIs across individual cells. Single cells of the same type were more similar to each other than to other types, but also displayed epigenetic heterogeneity. The method was further validated by comparing the CpG methylation pattern, methylation profile of CGIs/promoters and repeat regions and 41 classes of known regulatory markers to the ENCODE data. Although not every minor methylation differences between cells are detectable, scCGI-seq provides a solid tool for unsupervised stratification of a heterogeneous cell population.